1. Field of the Invention
The present invention relates to multilayer ceramic substrates and methods for manufacturing the same, and more specifically, to a multilayer ceramic substrate incorporating an element, such as a laminated ceramic capacitor, and to a method for manufacturing the same.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2002-84067 discloses a method for manufacturing a multilayer ceramic substrate incorporating a functional element by preliminarily preparing a functional element such as a capacitor element, inductor element, resistance element or the like, from a plate-like sintered body that is obtained by preliminarily baking a ceramic functional material, arranging the functional element inside an unsintered raw laminate which is to become a multilayer ceramic substrate by baking, and baking the raw laminate in this state.
Japanese Unexamined Patent Publication No. 2002-84067 also discloses application of a so-called non-shrink process for manufacturing the multilayer ceramic substrate described above. More specifically, outside constraining layers are arranged so as to sandwich the raw laminate to become a multilayer ceramic substrate in the direction of lamination. The outside constraining layers include ceramic material powder that will not be sintered at a baking temperature. Therefore, in a baking step, the outside constraining layers act to inhibit shrinkage of the laminate, and as a result, act such that non-uniform shrinkage is less likely to occur. The outside constraining layers are then removed after the baking step.
However, when the method described in Japanese Unexamined Patent Publication No. 2002-84067 is practiced, there sometimes arises a problem that cracking occurs in the incorporated functional element. This is caused by relatively large compression stress exerted on the functional element in the baking step for obtaining the multilayer ceramic substrate. In a so-called non-shrink process, while substantially no shrinkage occurs in the direction of a principal surface of the raw laminate at the time of baking, relatively large shrinkage occurs in the direction of thickness, and larger compression stress arises. In particular, an inner layer portion at a deeper position than a superficial portion of the laminate constrained by the outside constraining layers is less likely to receive the effect of the outside constraining layers, and thus is more likely to receive compression stress.
When a functional element is incorporated inside a raw laminate, a green sheet that forms the raw laminate is not usually able to deform to perfectly fit an outer surface of the functional element. Therefore, a gap often arises in the periphery of the functional element. Since such a gap largely shrinks at the time of baking, the stress concentrates in this region. This may result in a trouble that cracking occurs not only in the incorporated element but also in the multilayer ceramic substrate itself.
Japanese Unexamined Patent Publication No. 2002-84067 also discloses a via conductor, as one example of a wiring conductor on the side of the multilayer ceramic substrate which is to be electrically connected with a terminal electrode provided in the incorporated element. However, when a via conductor is directly connected with a terminal electrode of an incorporated element, there sometimes arises a problem in that cracking occurs undesirably in the incorporated element.
This is attributable to the fact that, in the baking step, a shrinkage factor of the material for the via conductor is smaller than that of the ceramic material forming the multilayer ceramic substrate, and the via conductor behaves so as to protrude toward the incorporated element at the time of baking. Probably, when a multilayer ceramic substrate is produced by a non-shrink process using outside constraining layers as described in Japanese Unexamined Patent Publication No. 2002-84067, in particular, the outside constraining layers act to push an end portion of the via conductor, and another end portion of the via conductor protrudes more extensively toward the incorporated element.
When the incorporated element has an electrode containing nickel, more specifically, when the incorporated element is, for example, a laminated ceramic capacitor and nickel is used as its internal electrode material, cracking may occur in a dielectric layer of the incorporated laminated ceramic capacitor starting from the internal electrode, or a terminal electrode of the laminated ceramic capacitor may be peeled off if baking is conducted under a condition suited for baking of the multilayer ceramic substrate.
The above problem is more significant in a case where silver is used in the wiring conductor on the side of the multilayer ceramic substrate. This is because, in using silver in a wiring conductor on the side of a multilayer ceramic substrate, an air atmosphere is generally employed as a furnace atmosphere at the time of baking. More specifically, when a raw laminate that is intended to become a multilayer ceramic substrate is baked in an air atmosphere, nickel contained in the internal electrode of the incorporated laminated ceramic capacitor is oxidized to generate NiO, and the volume of the internal electrode expands. This may cause cracking in the dielectric layer of the laminated ceramic capacitor or peeling of the terminal electrode of the laminated ceramic capacitor.